This invention relates to a method of measuring the density of material from a derivative absorption spectrum in magnetic field sweep.
One example of such a measurement is a measurement of a gas phase paramagnetic material employing electron paramagnetic resonance (hereinafter abbreviated to "EPR" when applicable).
The measurement of a gas phase paramagnetic material, especially NO and NO.sub.2, by an EPR device has been disclosed by Japanese Patent Application Laid Open No. 15489/1974. In the measurement according to the conventional technique disclosed by that application, a sample is placed in an EPR measurement sample tube, and the sample tube is set in an EPR cavity resonator. Thereafter, the EPR spectrum is measured by continuously sweeping the magnetic field while the microwave frequency is maintained unchanged. The density of the paramagnetic material, for instance NO or NO.sub.2, is determined from the intensity I of the EPR spectrum obtained. In this case, the period of time necessary for measurement depends on the magnetic field sweep time, and is, in general, relatively long, for instance several minutes. Accordingly, the density of the aimed material in the sample must be maintained unchanged at least for this period of time, and therefore the sample gas is kept in a closed system. Accordingly, the above-described conventional measurement is necessarily of the batch system, and is a non-continuous or intermittent measurement.
If the magnetic field is swept slowly for a period of time T starting at H.sub.0 as shown in FIG. 1(a), derivative peaks of the PSD output are observed at the inherent magnetic fields H.sub.1 and H.sub.2 of the aimed material as shown in FIG. 1(b), and the top peak value I.sub.1 and the bottom peak value I.sub.2 are observed thereat, respectively. The term "derivative peaks" as used herein merely refers to the peaks of the phase sensitive detector output in a conventional differential-type electron paramagnetic resonance (EPR) device. The difference value I between these two peak values I.sub.1 and I.sub.2 represents the density of the material. Accordingly, it takes a relatively long period of time T to detect a pair of peak values, and even if the density of a material is continuously varied as shown in FIG. 2, for instance, all that can be measured are incorrect intermittent values indicated by points a, b and c.